The present invention relates in general to maintaining an accurate steering angle position relative to a center steering angle of a motor vehicle, and more specifically to electric power assisted steering systems for vehicles wherein an electric motor operatively connected to a steering assembly includes position sensors for assisting in the determination of the steering angle.
It is well known to provide electric power assisted steering (EPAS) systems. A steering assembly typically comprises a hand wheel connected to a steering shaft which is operatively connected to one or more road wheels through a steering rack, although many different assemblies are in common use. Some EPAS systems use brushless electric motors with a motor position sensor to control the timing of switching (i.e., commutation of windings of the motor). The motor position sensor typically comprises an electromagnetic type switch or switches which change state whenever a magnet provided on the rotor passes the sensor. Alternatively, a magnetized disc can be mounted on the rotor shaft and the sensor may detect movement of the magnets on the disc.
A measure of vehicle steering angle position (i.e., the direction of the road wheels) relative to a straight-ahead or center steering angle is needed by various vehicle systems, such as suspension damper control systems, vehicle stability control systems, and lane guidance systems.
In a 3-phase brushless permanent magnet motor, for example, three Hall effect sensors can be located around the rotor in such a manner that a rough measurement of rotor electrical position can be obtained. However, this measurement alone is not sufficient for use as an indication of the position of the steering angle because 1) the electric motor goes through many complete electrical cycles as the steering system passes through its full range of motion, so the motor position output will repeat at different steering angles and thus produce an ambiguous signal (just as a steering shaft position sensor does), and 2) the angular resolution may be inadequate for any associated control systems using steering angle as an input (e.g., a vehicle stability control system).
When low resolution is not a problem, an EPAS motor Hall-effect sensor can be used in combination with a marker coupled to the steering system (e.g., a single position pulse at a reference position, such as the center position), as shown in U.S. Pat. No. 6,364,050, issued to Horton. To remove ambiguity from the EPAS motor position signal, transitions in the output of the motor position sensor are counted and the count is reset whenever an index signal is produced from the steering shaft sensor indicative of the steering assembly position being in the straight ahead (i.e., center) position for the vehicle. The index signal can be provided from a geared steering column sensor, a steering rack sensor, or a yaw rate sensor adapted to produce an output indicative of the yaw rate of the vehicle.
The counting of transitions is comprised of incrementing the count signal when the Hall-effect sensor output changes state corresponding to rotation in one direction, and decrementing the count signal when a change of state occurs corresponding to rotation in the opposite direction. The value of the count is therefore indicative of the angular position of the steering shaft relative to the known angular position at which the reset by the center position index marker occurs.
It is desirable to maintain the contents of the counter even when the vehicle ignition is turned off and the EPAS system would usually be de-powered. If the count was lost, then the absolute steering angle would not be known on the next driving cycle until an index signal was received. In addition, the steering angle may be changed while the ignition is off, resulting in an inaccurate count unless the sensor continues to be monitored. Therefore, the position sensing system included means adapted to “strobe” or periodically energize the motor position Hall-effect sensors and to sample the output of the sensing means when energized. The sensors may then be de-energized between samples (this is often referred to as sleep mode). This minimizes the average current drain compared to running the sensors continuously while enabling the counter to keep track of the steering shaft position even when the vehicle ignition switch is turned off without draining the vehicle battery excessively. A latch may be provided to latch the sampled value.
In the event of a dead main battery or other loss of power to the EPAS system, the latched value of the Hall counter may be lost, resulting in ambiguity of the absolute steering angle. After the dead battery is replaced, absolute position is nevertheless restored when the steering angle passes through the center position and the index marker is detected. It would be desirable, however, if system cost could be reduced by eliminating the index sensor and its associated wiring.
Where higher resolution measurement of steering angle is needed, it is known to utilize a high resolution angular position sensor either mounted directly on the steering shaft or connected thereto via a gear drive. This produces an output which does not depend on the EPAS gearbox ratio since it reads directly from the steering shaft, but is expensive to produce.
As shown in prior U.S. Pat. No. 6,354,396, issued to Horton et al, a motor position sensor can be used to assist a steering shaft position sensor in the determination of the position of the steering angle in order to overcome ambiguity in finding an absolute reference position. A motor position sensor comprising a Hall-effect sensor having a relatively coarse resolution is combined with a position sensor on the steering shaft. The motor is coupled to the steering shaft through a gearbox having a non-integer reduction gear ratio so that position signals from the two sensors are not synchronous and their relative phases identify the particular revolution of the steering shaft.
The resolution obtainable with Hall-effect sensors of reasonable cost may be insufficient for some motor control purposes. Prior application WO9908374A1, in the name of Wilson-Jones et al, teaches the use of a high resolution sensor on the steering shaft in order to increase the resolution of measuring the motor rotor position between the positions detected by a Hall-effect sensor. As previously mentioned, it would be an advantage to avoid the cost of a high resolution position sensor on the steering shaft.
Electronically commutated motors are known that internally incorporate position sensors that provide a higher resolution than can be easily achieved with Hall-effect sensors. For example, U.S. Pat. No. 5,625,239, issued to Persson et al, teaches an inductive sensor providing high resolution within each electrical cycle of motor rotation. Since the position sensor is formed integrally with the motor, it can be fabricated and installed more cheaply that a separate, steering shaft mounted high resolution position sensor. However, there are typically several electrical cycles within one full mechanical rotation of the rotor, so that the position derived from the inductive sensor is ambiguous with respect to the mechanical position. It is, however, sufficient for motor control purposes since the position within an electrical cycle is not ambiguous.